Re: [PATCH v2] mm/highmem: make kmap cache coloring aware

[Leonid Yegoshin]

On 07/21/2014 05:58 PM, David Rientjes wrote:
> On Thu, 17 Jul 2014, Max Filippov wrote:
>
> > From: Leonid Yegoshin <Leonid.Yegoshin@xxxxxxxxxx>
> >
> > Provide hooks that allow architectures with aliasing cache to align
> > mapping address of high pages according to their color. Such architectures
> > may enforce similar coloring of low- and high-memory page mappings and
> > reuse existing cache management functions to support highmem.
> Typically a change like this would be proposed along with a change to an
> architecture which would define this new ARCH_PKMAP_COLORING and have its
> own overriding definitions.  Based on who you sent this patch to, it looks
> like that would be mips and xtensa.  Now the only question is where are
> those patches to add the alternate definitions for those platforms?
Yes, there is one, at least for MIPS. This stuff can be a common ground 
for both platforms (MIPS and XTENSA)

> > Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@xxxxxxxxxx>
> > [ Max: extract architecture-independent part of the original patch, clean
> >    up checkpatch and build warnings. ]
> > Signed-off-by: Max Filippov <jcmvbkbc@xxxxxxxxx>
> > ---
> > Changes v1->v2:
> > - fix description
> >
> >   mm/highmem.c | 19 ++++++++++++++++---
> >   1 file changed, 16 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/highmem.c b/mm/highmem.c
> > index b32b70c..6898a8b 100644
> > --- a/mm/highmem.c
> > +++ b/mm/highmem.c
> > @@ -44,6 +44,14 @@ DEFINE_PER_CPU(int, __kmap_atomic_idx);
> >    */
> >   #ifdef CONFIG_HIGHMEM
> >   
> > +#ifndef ARCH_PKMAP_COLORING
> > +#define set_pkmap_color(pg, cl)		/* */
> This is typically done with do {} while (0).
>
> > +#define get_last_pkmap_nr(p, cl)	(p)
> > +#define get_next_pkmap_nr(p, cl)	(((p) + 1) & LAST_PKMAP_MASK)
> > +#define is_no_more_pkmaps(p, cl)	(!(p))
> That's not gramatically proper.
>
> > +#define get_next_pkmap_counter(c, cl)	((c) - 1)
> > +#endif
> > +
> >   unsigned long totalhigh_pages __read_mostly;
> >   EXPORT_SYMBOL(totalhigh_pages);
> >   
> > @@ -161,19 +169,24 @@ static inline unsigned long map_new_virtual(struct page *page)
> >   {
> >   	unsigned long vaddr;
> >   	int count;
> > +	int color __maybe_unused;
> > +
> > +	set_pkmap_color(page, color);
> > +	last_pkmap_nr = get_last_pkmap_nr(last_pkmap_nr, color);
> >   
> >   start:
> >   	count = LAST_PKMAP;
> >   	/* Find an empty entry */
> >   	for (;;) {
> > -		last_pkmap_nr = (last_pkmap_nr + 1) & LAST_PKMAP_MASK;
> > -		if (!last_pkmap_nr) {
> > +		last_pkmap_nr = get_next_pkmap_nr(last_pkmap_nr, color);
> > +		if (is_no_more_pkmaps(last_pkmap_nr, color)) {
> >   			flush_all_zero_pkmaps();
> >   			count = LAST_PKMAP;
> >   		}
> >   		if (!pkmap_count[last_pkmap_nr])
> >   			break;	/* Found a usable entry */
> > -		if (--count)
> > +		count = get_next_pkmap_counter(count, color);
> And that's not equivalent at all, --count decrements the auto variable and
> then tests it for being non-zero.  Your get_next_pkmap_counter() never
> decrements count.
David, the statements

            count = get_next_pkmap_counter(count, color);
            if (count > 0)

are extended in STANDARD (non colored) case to

            count = (count - 1);
            if (count > 0)

which are perfect equivalent of

            if (--count)

> > +		if (count > 0)
> >   			continue;
> >   
> >   		/*

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